1. Field of the Invention
The present invention relates to a copying machine for copying an image of an object onto a recording material, and more specifically, to a copying machine capable of providing a particular blank area in a copied image based on the result of a comparison of an original and a sheet with respect to size.
2. Description of the Related Art
An image forming apparatus such as an electrostatic copying machine including an image reading portion for reading the image of an object to be copied, an image forming portion for forming a copying image based on the image read out by the image reading portion, and a sheet carrying portion for feeding a copy sheet on which the image formed by the image forming portion is stored.
The image reading portion includes an original document table on which an object is placed, and a document cover for pressing the document positioned on the document table in place. The document cover having a cover portion integrated into the cover, used for ensuring the uniform distribution of the white background of an image obtained by copying.
The document cover is formed to be usable even when the top is opened, so that objects having shapes other than that of a sheet, such as a book or a three-dimensional object can be subjected to the copying operation.
The image forming portion includes a photosensitive member on which an electrostatic latent image is formed in accordance with the image read out, a developing device for supplying toner on the latent image formed on the photosensitive member, and a copy starting switch for instructing the start of formation of the image.
In the case where the object is a book or a three-dimensional object, it commonly happens that an undesirable black margin (toner image) occurs in the background, that is, around the copied image, since the document cover is released or is not closed completely. In the case where a copy sheet larger than the size of an image defined in accordance with an object to be copied or a copying magnification is used, a black margin of a similar type can occur around the image formed on a sheet, since the cover is opened.
In order to remove such a black margin, a technique of covering the periphery (the area to be blank) of the object in advance with blank paper has been proposed.
Further, in accordance with an increase in the number of functions of the copying machine, a number of optional functions have recently been proposed, such as a partial erase function for erasing a black margin, and a trimming function for copying only the image data of the necessary image area.
However, even in the case where the partial erase function or the trimming function is provided, such a function is, in practice, rarely used, since the operation of specifying (inputting) the area to be erased (trimmed) is complicated. Moreover, with the manual paper feeding mode, the black margin cannot be erased in many cases, since the length of the sheet cannot be identified until a required length of at least one sheet has been fed into the copying machine.
Therefore, even with the copying machine having the partial erase function or trimming function, when a book or a three-dimensional object is subjected to copying (or when a sheet larger than the image obtained by copying is used), a black margin is likely to occur in the peripheral region of the copied image, thereby causing unnecessary consumption of the toner contained in the developing device. As a result, the replenishment of the toner in the developing device cannot keep level with its consumption, resulting in a decrease in the image density of copied images in subsequent operations. Further, the number of copies which can be achieved with a certain amount of toner is decreased, thereby increasing the cost of copying as a whole.
Moreover, the technique of covering the peripheral region of an object with blank paper results in an decrease in the efficiency of the copying operation, and excessive consumption of blank paper, which is unnecessary in recording of an image.